Pulsed NdYAG lasers are widely used in industrial processes such as welding, cutting and marking. Care has to be taken in these processes to ensure that the plasmas generated by the laser do not interfere with the incoming laser pulses. The relatively low pulse repetition rates (6 kHz) at high peak powers that are achievable in a NdYAG laser have led to their wide application in laser machining.
Fibre lasers are increasingly being used in industry for the processing of materials by, for example, the welding, cutting and marking of the materials. The advantages of fibre lasers include high efficiency, robustness and high beam quality. Examples include femtosecond lasers for multiphoton processing such as the imaging of biological tissues, Q-switched lasers for machining applications, and high-power continuous-wave lasers. The disadvantage of the fibre lasers is their relatively low energy storage capacity as compared to NdYAG lasers. A relatively higher energy storage capacity is an advantage because it allows higher energy pulses to be released from the laser in Q-switched applications.
In many instances, fibre lasers need to compete with the more mature diode pumped solid state lasers. In order to do so, much greater optical powers need to be achieved, with high reliability and lower cost.
Fibre lasers are typically longer than diode-pumped solid state lasers, and this leads to non-linear limitations such as Raman scattering becoming problematical. It would be advantageous to have fibre lasers that are shorter.
Fibre lasers are typically pumped with diode lasers in bar or stack form. The output from bars and stacks is not ideally matched to the geometry of fibre lasers, leading to a loss in brightness, and thus the need to increase the length of cladding pumped fibre lasers in order to obtain the necessary absorption and output energy. Fibre lasers are increasingly being pumped with single-emitter laser diodes whose outputs are combined together.
Fibre lasers are also competing with solid state disk lasers in the industrial processing of many materials. Power levels of several kilowatts are often required, and in many instances control of beam quality, efficiency and/or the beam profile would give the fibre laser advantages over the disk laser.
In certain processes, such as the cutting of metal, there are advantages in combining lasers with a gas such as oxygen, nitrogen or a noble gas. U.S. Pat. Nos. 5,220,149, 5,609,781, 5,747,771, 6,118,097, 6,288,363, 6,376,797 and 6,423,928, which are hereby incorporated herein by reference describe various applications where gases and lasers are utilized together. In one example, a laser nozzle directs both a laser beam and a flow of oxygen gas onto sheet metal. The laser beam heats the metal to initiate cutting and the oxygen gas acts as a cutting gas to cut into or through the metal. This process requires a high energy laser beam to heat the metal. An advantage would be gained by reducing the energy of the laser beam that is required without dependence on diffractive or holographic optics. This would be especially useful for high power fibre lasers since these are generally much less coherent than conventional solid-state lasers. Linewidths of 1 nm to 5 nm are common in high power fibre lasers. A further advantage would be gained by providing a fibre delivery system which would simplify the system required to direct the laser nozzle.
An aim of the present invention is to provide apparatus for the industrial processing of a material by optical radiation that reduces the above aforementioned problems.